1. Field of the Invention
The present invention relates to an electro-rheological fluid, more particularly to an electro-rheological fluid comprising non-conductive media, watersoluble starch as a conductive particle and less than 5 wt % water.
2. Description of the Background Art
Generally, Electro-rheological (ER) fluid refers to a fluid whose mechanical property is variable by applying an electric field. Basically, it is a colloidal fluid comprising highly conductive particles and viscous non-conductive media, wherein the conductive particles are dispersed in the non-conductive media. When an external electric field is applied to an ER fluid, the yield stress and dynamic viscosity of the ER fluid increase abruptly and reversibly. The ER fluid quickly reacts on the electric field, but is also capable of having a reversible reaction against the appliance of the electric field, which is known as an ER effect.
In FIGS. 1A and 1B, the microscopic mechanism of the ER fluid is presented.
As shown in FIG. 1A, when no electric field is applied, the ER fluid 101 exhibits a Newtonian fluid property, i.e. conductive particles 103 flow together with a media 102 within an electrode 104. As shown in FIG. 1B, when the electric field is applied, the ER fluid 101 displays a Bingham behavior wherein the yield stress increases as an external electric field is applied.
The initial ER fluid, developed at the end of the 19th century, was composed of only a liquid, but it failed to give a satisfactory result (Duff, A. W., Physical Review, Vol. 4, No. 1, 23 (1986)).
Later, a solid dispersion system was first proposed by Winslow, which had given a considerable progress in the field of ER fluid [Winslow, W. H., J. of Applied Physics, Vol. 20, 1137(1949)]. Since then, researches have been conducted for a system comprising conductive particles and non-conductive media.
As the conductive particles, silica gel, water-soluble starch or a semiconductor material has been used.
It is well known that ER fluid comprising a water-soluble starch, which is known as an aqueous material, as conductive particles displays the ER effect only if it contains at least 5 wt % of water. That is, if the water content of the ER fluid comprising the water-soluble starch is less than 5 wt %, it cannot be used as an ER fluid because its reversibility rapidly degrades. However, the water contained in ER fluid can cause the corrosion of device, restrict its operation temperature, and incur high power consumption. Therefore, there has been an intense study on the development of ER fluid having the reduced water content has been intensively tried.
Therefore, an object of the present invention is to provide an ER fluid comprising a dried water-soluble starch as conductive particles and which has an excellent ER effect even though the ER fluid contains less than 5 wt % of water.
To achieve these and other advantages, the present invention also provides a method for producing an ER fluid, comprising the steps of: grinding water-soluble starch particles in a grinder so as to render the particle size less than 10 xcexcm; drying the thusly obtained water-soluble starch particles in a thermohygrostat chamber in a temperature of 35xcx9c45xc2x0 C. and relative humidity of 30xcx9c50%; mixing the dried water-soluble starch particles and a non-conductive media in a container at a proper weight ratio; boiling the obtained fluid in an oil bath of 100xcx9c150xc2x0 C.; grinding the obtained ER fluid in a grinder so as to homogenize and evenly mix the particles.